The ubiquitous beta-herpes virus CMV exhibits a number of natural adaptations - including 1) elicitation and long-term maintenance of uniquely strong (mucosally-oriented) cellular and humoral immunity;2) lifelong persistence despite this robust immune response, 3) the ability to sub-clinically re-infect and immunologically boost fully immune hosts, and 4) little pathogenicity in normal hosts - that make it a provocative candidate for development as a vaccine vector for chronic, elusive lentiviral pathogens like HIV/SIV (for which protection will likely require potent, long-lasting cellular and humoral immunity). The overall goal of this proposal is to use the rhesus macaque (RM) model (RhCMV/SIV) to explore the hypothesis that the special adaptations of CMV vectors (immunogenicity, persistence, ability to re-infect immune hosts) can quantitatively and/or qualitatively improve on the immunogenicity of current vaccine strategies, and as a result, safely elicit protective lentivirus-specific immunity. Our preliminary data indicates that RhCMV can be genetically manipulated to express SIV Ags, and that such recombinant RhCMV can re-infect RhCMV- "immune" RM and elicit a de novo immune response. This project will follow up on these promising preliminary results by completion of the following Specific Aims: Specific Aim 1 will construct sophisticated RhCMV vectors encoding codon-optimized SIV gag, env (native and modified for proteosome degradation), and a (detoxified) rev/nef/tat chimera under the control of promoters of different kinetic classes, and assess the level and pattern of SIV Ag expression, cell tropism, and growth of these vectors in vitro. Specific Aim 2 will compare the in vivo growth, immunogenicity (including CD4 and CD8 T cell responder frequencies, breadth of epitope recognition, tissue distribution, functional avidity, and functional differentiation, and for Ab, neutralization capability) and protective capacity of these different vectors, and will determine the relative effectiveness of concurrent vs. sequential administration. Specific Aim 3 will evaluate the relative effectiveness of mucosal (vs. peripheral) routes of immunization, and of DNA priming prior to RhCMV vector administration. Specific Aim 4 will compare the ability of an optimized CMV vector-based vaccine protocol vs. a conventional DNA/vaccinia prime-boost strategy to control pathogenic SIV challenge, explore mechanisms for vaccine failure, and determine the immunologic parameters predictive of protection.